In the drilling of bolt holes for reinforcing structure in mine roofs, it is often necessary to penetrate to a depth greater than the heighth of the mine chamber below the roof. Various systems have been developed for accomplishing this task, such as that shown in Hansen, et al. U.S. Pat. No. 4,009,760. Typically, such systems comprise a starter-driver rod adapted to be releasably attached to a drill bit, and one or more extension rods adapted to be successively interposed between the bit and the starter-driver rod as the drilling of the hole progresses.
In such systems, as well as in those employing only a driver rod and a drill bit, it is important to be able to quickly and easily remove the bit from the driver rod. At the same time, however, the coupling between the bit and driver rod must be sufficiently strong to transmit the relatively high torque required to drill holes in mine roofs, as well as provide a locking mechanism to preclude removal of the bit from the driving rod during drilling of the hole. Heretofore, couplings between the drill bit and the driver rod have not proved entirely satisfactory in this regard. For example, the coupling shown in the above-cited Hansen patent comprises a female hex socket and mating male hex plug with a removable pin adapted to be received within a transversely extending hole through the socket wall and plug. While this system has proved satisfactory from an operating standpoint, it is not particularly conducive to quick release operation.
Other coupling systems presently offered by manufacturers of drill steel are various modifications of the basic idea of providing a male and female hex with detent structure for interlocking the two. Such systems are relatively expensive to manufacture and many require special tools for releasing the detent.